The present invention relates to a TV antenna apparatus for vehicles which is usually called a mast-less antenna apparatus having no masts protruded from the car body but a sheet antenna for receiving a TV broadcast wave.
Recently there have been great demands for the use of the mast-less antenna apparatus as a TV antenna apparatus for vehicles.
FIG. 21 is a view showing an example of a wideband TV antenna apparatus 20 conventionally used for receiving a TV broadcast wave. As shown in FIG. 21, the apparatus 20 includes a folded antenna element 21 obtained by folding a metal pipe with a diameter of about 10 mm like an oval or a loop. A feed section 23 is provided in the central part of the antenna element 21 and connected to a feeder 25, which is constituted of a coaxial cable, through a balun 24 (matching transformer for choosing between balanced and unbalanced lines).
The folded antenna element 21 is formed of a so-called rigid member, and its length is about 1300 mm and its width is about 50 mm when it is used in the VHF band (falling between 90 MHz and 222 MHz in Japan). Since the antenna element 21 so constituted is too large in size and rigid, it is not adapted to an object so well. For this reason, the antenna element 21 is not suitable to be used as a mast-less antenna apparatus which is mounted on a resin panel of a vehicle.
The antenna element 21 constituted as shown in FIG. 21 and used in the VHF band is not able to receive a radio wave of the UHF band with efficiency. In order to satisfactorily receive the radio wave of the UHF band as well as that of the VHF band, a small-sized antenna element for the UHF band, which is about one-fifth of the antenna element 21, has to be added and, in this case, in order to prevent the antenna element 21 for the VHF band and that for the UHF band from interfering with each other, a wave separator for separating the radio waves of both the antenna elements is needed.
As an example of the mast-less antenna apparatus, there is an antenna apparatus used for a windowpane of a vehicle. In this antenna apparatus, a thin, narrow, strip conductor provided on the rear window, is used as an antenna element.
The vehicle windowpane antenna apparatus is suitable as the mast-less antenna apparatus because no portions are greatly protruded from the car body. However, even though, for example, a four-channel space diversity reception method is adopted, no good space diversity effect can be obtained.
To obtain good space diversity effect, the antenna elements have to be arranged at intervals of about .lambda./4 (.lambda.: wavelength). Assuming that the frequency f of received wave is 100 MHz, .lambda./4 is 75 cm. Thus, the antenna elements need to be arranged vertically and horizontally on the rear window of a vehicle (especially an automobile) at intervals of 75 cm. It is however physically difficult to arrange the antenna elements on the rear window in the same manner as described above.
The radio waves are coming from different directions every moment in accordance with the movement of the vehicle. Therefore, when the antenna elements of the vehicle windowpane antenna apparatus are formed on the rear window of the vehicle, they can favorably receive the wave coming from a direction perpendicular to the window surface, but not the waves falling upon the sides and front of the vehicle because they are obstructed by the car body.